A hydrogel is a hydrated cross-linked polymeric system that contains water in an equilibrium state. Hydrogels typically are oxygen permeable and biocompatible, making them a preferred material for producing biomedical devices and in particular contact or intraocular lenses.
Conventional hydrogels are prepared from monomeric mixtures predominantly containing hydrophilic monomers, such as, 2-hydroxyethyl methacrylate or N-vinyl pyrrolidone. U.S. Pat. Nos. 4,495,313; 4,889,664 and 5,039,459 disclose the formation of conventional hydrogels. Displaceable diluents are often used to form conventional hydrogels, such as water-displaceable boric acid esters of polyhydric alcohols. In U.S. Pat. No. 4,680,336, the displaceable diluents disclosed are water-displaceable organic compounds selected on the basis of their viscosity and their Hansen cohesion parameters relative to the cohesion parameters of the polymeric component of the hydrogel to be prepared. Oxygen permeability of these conventional hydrogel materials relates to the water content of the materials, and is typically below 20-30 barrers. For contact lenses made of the conventional hydrogel materials, that level of oxygen permeability is suitable for short-term wear of the contact lenses; however, that level of oxygen permeability may be insufficient to maintain a healthy cornea during long-term wear of contact lenses (e.g. 30 days without removal). Therefore, efforts have been made and continue to be made to increase the oxygen permeability of conventional hydrogels.
One known way to increase the oxygen permeability of the hydrogels is to add silicone-containing monomers to the hydrogel formulations, thereby making silicone hydrogels. Silicone-containing polymers generally have higher oxygen permeabilities than conventional hydrogels. Silicone hydrogels have been prepared by polymerizing mixtures containing at least one silicone-containing monomer and at least one hydrophilic monomer. Either the silicone-containing monomer or the hydrophilic monomer may function as a crosslinking agent (a crosslinking agent is a monomer having multiple polymerizable functionalities) or a separate crosslinking agent may be employed. The formation of silicone hydrogels has been disclosed in U.S. Pat. Nos. 4,954,587; 5,010,141; 5,079,319; 5,115,056; 5,260,000; 5,336,797; 5,358,995; 5,387,632; 5,451,617; and 5,486,579; and WO 96/31792. In these references, n-hexanol, ethanol, and n-nonanol are used as diluents to compatibilize the silicone monomers and the hydrophilic monomers. Additional broad classes of materials are disclosed in those patents as potentially useful as diluents. The diluents used in the prior art were used in relatively large amounts of the total weight of the reaction mixture. A large amount of the diluent makes it difficult to mold the silicone hydrogel lenses reproducibly, because a large amount of the diluent leads to a large amount of diluent evaporating during the process. Further, the use of such a large amount of diluent may be a fire hazard, and often leads to polymeric materials with reduced toughness. If molding, the use of a large amount of diluent may also make it necessary to mold the polymer in a larger sized mold to compensate for the shrinkage caused by the removal of the diluent after polymerization. More importantly, the diluents used in the prior art inadequately solubilize many blends of silicone monomers and macromers and hydrophilic monomers, especially blends with relatively high levels of hydrophilic monomers. These blends and resulting polymers are opaque and not useful for contact lenses.
An alternative approach to forming silicone-hydrogels which avoids the addition of diluents is disclosed in U.S. Pat. Nos. 5,321,108, 5,387,662 and 5,539,016. These patents describe the use of polysiloxanes with a polar fluorinated graft or side group having a hydrogen atom attached to a terminal difluoro-substituted carbon atom. While this does improve compatibility for certain combinations of silicone and hydrophilic monomers, it requires multiple step synthesis of complex silicone macromers.
There still remains a need in the art for silicone hydrogels which are polymerized in an economic and efficient way.